scaling: include padId in socketio query string

src/tests/backend/specs/socketio.js

@@ -52,12 +52,13 @@ const connect = async (res) => {
       ([name, cookie]) => `${name}=${encodeURIComponent(cookie.value)}`).join('; ');
 
   logger.debug('socket.io connecting...');
+  const padId = res.req.path.split('/p/')[1];
   const socket = io(`${common.baseUrl}/`, {
     forceNew: true, // Different tests will have different query parameters.
     path: '/socket.io',
     // socketio.js-client on node.js doesn't support cookies (see https://git.io/JU8u9), so the
     // express_sid cookie must be passed as a query parameter.
-    query: {cookie: reqCookieHdr},
+    query: {cookie: reqCookieHdr, padId},
   });
   try {
     await getSocketEvent(socket, 'connect');